"We are a girl and a boy."
Translation:Nous sommes une fille et un garçon.
Im not entirely sure what youre asking. If youre talking about the way they sound, une has a longer "u" sound and un has a shorter sound. Another difference between the two is the gender they represent. Une is for feminine words, and un is for masculine words. With fille, garcon, homme, and femme it is pretty easy to work out the correct gender. outside of these four, you just have to remember what gender a word is, as to wether to use une or un.
All nouns in French are gendered - either masculine or feminine, as are the articles they use → un for masculine nouns and une for feminine nouns. It is advisable to learn the article along with the noun and its meaning as you progress. You'll find life a lot easier if you do that.